Relax the via header check

The via header can have multiple space separated entries, if it hits
the staging server and the prod server. So relaxes the exact equality
check.

Bug: 960105
Change-Id: Ib50ee751f40b0892376a4ae622f0ac061efb401d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600632
Commit-Queue: rajendrant <rajendrant@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657743}

tools/chrome_proxy/webdriver/common.py

@@ -869,8 +869,8 @@ class HTTPResponse:
     return self._redirect_chain
 
   def ResponseHasViaHeader(self):
-    return 'via' in self._response_headers and (self._response_headers['via'] ==
-      self._flags.via_header_value)
+    return 'via' in self._response_headers and (self._flags.via_header_value in
+      [piece.strip() for piece in self._response_headers['via'].split(',')])
 
   def WasXHR(self):
     return self.request_type == 'XHR'